What is desertification?

A) when a desert changes to some other environment
B) when a desert suffers increased erosion due to loss of plants
C) when grasslands or some other ecology is converted into a desert
D) the deposition of one layer of desert sand over another

Desertification refers to the process by which fertile land becomes desert, typically as a result of drought, deforestation, or inappropriate agriculture. This usually involves the transformation of grasslands, croplands, or other types of landscapes into desert-like conditions, not the other processes described in the other options.